In wireless communications, users situated relatively far from a base station that serves them are generally more susceptible to interference from neighboring base stations and to signal attenuation. As a consequence, such users may experience relatively low signal-to-interference-and-noise ratios (SINRs). The relatively distant users are referred to as “cell edge users” or as users with “poor geometry.” It will be understood that when one user is said to be more “distant” from the base station than another, what is meant does not depend solely on geographical distance, but also to susceptibility to other factors leading to attenuation and interference.
Several techniques with different degrees of complexity can be considered for out-of-cell interference mitigation in orthogonal frequency division multiple access (OFDMA) systems. Most of these schemes involve transmitting in any given cell over a portion of the spectrum that is smaller than the entire available bandwidth while neighboring cells employ a different portion of the spectrum. The primary goal of many of these techniques is to enhance the cell edge users' spectral efficiency and throughput at the expense of average sector throughput, or in some cases, with improved sector throughput as well. The interference mitigation schemes differ primarily in how the bandwidth occupied and transmitted power in a given sector are determined, and whether it is adapted as a function of the users being served, and whether it is coordinated across different cells.
For example, some schemes involve coordinated scheduling across multiple cells, in which bandwidth allocations are jointly determined across the cells on a per scheduling interval basis. Such schemes typically involve significant signaling between base stations, thus making them difficult to implement. Other schemes are based on static frequency partitioning or distributed coordination that do not involve signaling between base stations.
In an OFDMA system the transmission band is divided into a number of sub-carriers and information is transmitted by modulating each of the sub-carriers. Furthermore, time is divided into slots consisting of a number of OFDM symbols and transmissions are scheduled to users by assigning frequency resources (e.g., a set of logical sub-carriers) on specific slots. The logical sub-carriers are mapped to physical sub-carriers for transmission. The mapping can change from time to time and is referred to as frequency hopping. Frequency hopping is done to achieve interference averaging.
In OFDMA systems that support fractional frequency reuse (FFR) for interference mitigation, frequency and time resources (within an interlace period or frame that repeats periodically) are divided into several resource sets. Frequency hopping of sub-carriers is restricted to be within the sub-carriers of a resource set so that users scheduled on a certain resource set experience interference only from users scheduled in neighboring sectors in the same resource set. Each resource set is typically reserved for a certain reuse factor and is associated with a particular transmission power profile. For example, resource set 1 may be reserved for good geometry users in all sectors and resource set 2, 3, 4 for bad geometry users in sectors 1, 2, 3, respectively, in each cell resulting in ⅓ reuse for bad geometry users and universal reuse for good geometry users. Thus, fractional frequency reuse is achieved.
Fixed fractional frequency reuse does not adapt to traffic dynamics in the sense that the frequency reuse achieved is not adjusted based on interference conditions experienced by the users. Instead of a fixed partition of the available bandwidth leading fixed frequency reuse, it is possible to achieve dynamic frequency reuse through prioritized use of sub-carriers in the adjacent sectors. Interference avoidance is achieved by assigning different priority orders in the neighboring sectors so that when transmitting to cell edge users using sub-channelization, the neighboring interfering sectors transmit on different sub-carriers. Such a scheme was described in Das et al., “Interference mitigation through intelligent scheduling”, Proc. Asilomar Conference on Signals and Systems, Asilomar, Calif., November 2006. For such an approach it is necessary to do frequency planning to determine the priorities and also dynamically adjust the thresholds under which users are assigned to different resource sets.
It is known that reuse patterns, such as the 1:3 reuse pattern of FIG. 1, are useful for reducing interference between neighboring sectors. As seen in FIG. 1, each cell (e.g., shown as hexagonal units 102, 104, 106) of a network is divided, according to a 1:3 reuse pattern, into respective α, β, and γ sectors. The three classes of sectors are geographically distributed in such a way that, ideally, two sectors of the same class (e.g., characterized by the same subcarrier frequency) do not share a common edge. As a consequence, interference can be reduced by assigning mutually disjoint sets of subcarriers to the respective sector classes. In such a situation, the three sector classes may be said to be “mutually orthogonal” with respect to their use of subcarriers. In the figure, the respective sets of sub-bands, each of which contains sets of subcarriers, are denoted as f1, f2, and f3.